In the past a tremendous amount of work has been devoted to the development of particles, in particular catalytically active particles, for many different processes. There has also been a considerable effort to try to understand the advantages and sometimes disadvantages of effects of shape when deviating from conventional shapes such as pellets, rods, spheres and cylinders for use in catalytic as well as non-catalytic duties.
Examples of further well-known shapes are rings, cloverleafs, dumbells and C-shaped particles. Considerable efforts have been devoted to the so-called “polylobal”-shaped particles. Many commercial catalysts are available in TL (Trilobe) or QL (Quadrulobe) form. They serve as alternatives to the conventional cylindrical shape and often provide advantages because of their increased surface-to-volume ratio which enables the exposure of more catalytic sites thus providing more active catalysts.
An example of a study directed to effects of different shapes on catalytic performance can be found in the article by I. Naka and A. de Bruijn (J. Japan Petrol. Inst., Vol. 23, No. 4, 1980, pages 268-273), entitled “Hydrodesulphurisation Activity of Catalysts with Non-Cylindrical Shape”. In this article experiments have been described in which non-cylindrical extrudates with cross-sections of symmetrical quadrulobes, asymmetrical quadrulobes and trilobes as well as cylindrical extrudates with nominal diameters of 1/32, 1/16 and 1/12 inch were tested in a small bench scale unit on their hydrodesulfurization activity (12% wt MoO3 and 4% wt CoO on gamma alumina). It is concluded in this article that the HDS activity is strongly correlated with the geometrical surface-to-volume ratio of the catalyst particles but independent of catalyst shape.
In EP-0,220,933, it is described that the shape of quadrulobe-type catalysts is important, in particular with respect to a phenomenon known as pressure drop. From the experimental evidence provided it appears that asymmetric quadrulobes suffer less from pressure drop than the closely related symmetrical quadrulobes. The asymmetrically shaped particles are described in EP-0,220,933 by way of each pair of protrusions being separated by a channel which is narrower than the protrusions to prevent entry thereinto by the protrusions of an adjacent particle. It is taught in EP-0,220,933 that the shape of the particles prevents them from “packing” in a bed causing the overall bulk density of the catalyst bed to be low.
EP-0,428,223 discloses that the catalyst particles may be in the form of cylinders; hollow cylinders, for example cylinders having a central hollow space which has a radius of between 0.1 and 0.4 of the radius of the cylinder; straight or rifled (twisted) trilobes; or one of the other forms disclosed in U.S. Pat. No. 4,028,221. Trilobe extrudates are said to be preferred.
EP-0,218,147 discloses a helical lobed, polylobal extrudate particle having the outline shape of three or four strands helically wound about the axis of extrusion along the length of the particle and its use as a catalyst or catalyst support, in particular as a catalyst or catalyst support in hydrotreating operations. The helical shape of the catalyst is said to reduce the pressure drop across fixed bed reactors through which liquid and/or gas reactants are passed. In this way, smaller catalyst particles can be employed in a given reactor design to meet the pressure drop requirements.
Since many of the findings in the art are conflicting and pressure drop problems continue to be in existence, especially when surface-to-volume ratios are increased by reducing particle size, there is still considerable room to search for alternative shapes of (optionally catalytically active) particles which would diminish or even prevent such problems. When using a process employing a fixed bed of catalyst particles, a major consideration in the design of the process is the pressure drop through the catalyst bed. It is most desirable that the pressure drop should be as low as possible. However, it is well reported in the art that, for a given shape of catalyst particles, as the size of the catalyst particles in a fixed bed is reduced, there is a corresponding increase in pressure drop through the catalyst bed. Thus, there exists a conflict in the design of fixed catalysts beds when trying to achieve a satisfactory level of catalyst efficiency whilst keeping the pressure drop through the bed to a minimum. In addition to the above, the catalyst particles should be sufficiently strong to avoid undesired attrition and/or breakage. Especially in fixed beds the bulk crush strength should be (very) high, as beds are used in commercial reactors of up to 15 meters high. Especially at the lower end of the bed the pressure is very high and the strengths of the catalyst particles plays an important part. This is an additional complication in designing further improved catalyst particles. A still further complicating element is the manufacturing process of catalyst particles. There is a need for a fast, relatively inexpensive and suitable manufacturing process which will enable the production of catalyst particles in large quantities. One example of such a commercially available manufacturing process is an extrusion process. It is taught in the prior art that the efficiency of a catalyst in general increases as the size of the catalyst particle decreases. Further, catalysts should show a high stability, i.e. deactivation should be very low.